One particular area of interest with regard to bone cements or calcium phosphate compositions focuses on the addition of viscosity-modifying agents to make the compositions liquid-like or flowable. Flowability is a consideration especially when the bone cements or calcium phosphate compositions are used as an aid to filling voids in bone, to treating bone defects, or to augment the stability of other implantable devices in vivo. Without adequate flowability, bone cements typically are molded by hand to fit a particular bone void or defect. However, non-flowable cements cannot be used when it is necessary or desired to fill a hole, void, or defect in a bone larger than the area from which a surgeon can access the hole, void, or defect. In addition, non-flowable cements cannot be easily created in situ to fill a particular hole, void, or defect, but must be prefabricated. If the molded bone cement does not fit or cannot be forced to do so by external pressure of a surgeon once relatively in place, it does not create a sufficient implantable construct to facilitate healing and/or bone regrowth.
Another particular area of interest with regard to bone cements or calcium phosphate compositions focuses on reinforcing its bone cements with fibers, usually relatively long fibers, or fiber meshes. Much of the prior art in implantable composite materials focuses on a strong and resilient matrix impregnated with reinforcing filler particles, whiskers, or meshes. Often the ceramic bone cements are strong enough but are brittle and not sufficiently resistant to catastrophic failure (e.g., through cracking) to function as the matrix material. Polymers, usually resorbable, generally perform that reinforcing function. Resorbable implant materials, such as polylactides and polyglycolides, as compared to traditional, non-resorbable metal or composite materials, for example, have the advantage of being biocompatible, of being biodegradable after a period of time, and of not requiring removal, e.g., in bone fixation or repair applications. These qualities are especially important for implant matrices that are designed to be temporary place fillers (and in some cases, stabilizing components) for healing and/or regrowth, e.g., of bone voids or defects.
In addition, with most composites, the reinforcing material is different than the matrix material, in the hopes that the most beneficial set of properties can be amplified from each component, while the less desirable characteristics of each component are preferably reduced. As a result, the fibers used are generally ceramic in nature or of a (co)polymer composition of different chemistry. However, there is little prior art addressing implantable materials containing resorbable or biodegradable fibers in a ceramic matrix. One example of fiber-reinforced ceramic matrices can be found in a Xu et al. article entitled “Reinforcement of a Self-Setting Calcium Phosphate Cement with Different Fibers,” in J. Biomed. Mater. Res., 2000, vol. 52, pp. 107-114 (“the Xu article”).
The Xu Article discloses water-based calcium phosphate cements that were reinforced with fibers of aramid (KEVLAR), carbon, E-glass, and POLYGLACTIN. It discloses fiber lengths of 3 mm, 8 mm, 25 mm, 75 mm, and 200 mm, with fiber volume fraction loadings of 1.9%-9.5% in CPC powder, which contains a mixture of tetracalcium phosphate and anhydrous dicalcium phosphate, which react in an aqueous environment to form hydroxyapatite. The POLYGLACTIN fibers in the Xu article are 90/10 copolymers of glycolide/lactide and had a measured diameter of about 200 microns.
It is, therefore, desirable to obtain a fiber-reinforced and/or flowable calcium salt-containing composite material for implantation that exhibits improvements in key mechanical properties as a result of a specific combination of properties of the ingredients, particularly fiber length, fiber diameter or width, fiber aspect ratio, flow additive incorporation, continuous fiber/stent/mesh incorporation, or the like, or a combination of multiple variables.